Prolly the most effective way is to add a 10oz flywheel weight. If you do the work yourself, it's fairly cheap. You could get the bolt-on kind and that would make it easier to install.
Moose also has a reed block spacer that will add more low end to the bike. Also, adjusting the timing, running thicker base gasket, and a torque pipe will help.
More expensive is to have it ported for more low to mid.